Membrane structure and method of making

ABSTRACT

A membrane structure is provided. The membrane structure includes a first layer having a plurality of pores; and a second layer disposed on, the first layer. The second layer has a plurality of unconnected pores. At least a portion of the plurality of unconnected pores of the second layer is at least partially filled with a filler such that the first layer is substantially free of the filler. At least a portion of the plurality of unconnected pores of the second layer is in fluid communication with at least one of the pores of the first layer. A method of making a membrane structure is provided. The method includes the steps of providing a first layer having a plurality of interconnected pores; disposing a second layer on the first layer, and filling at least a portion of the unconnected pores of the second layer with a filler such that the first layer is substantially free of the filler. Disposing a second layer includes depositing a metal layer on the first layer; and anodizing the metal layer to convert the metal layer into porous oxide layer.

BACKGROUND

The invention relates generally to a membrane structure. Moreparticularly, the invention relates to a membrane structure having highflux and high selectivity. The invention also relates to a method ofmaking a membrane structure.

Porous membrane structures have been extensively used in filtration,separation, catalysis, detection, and sensor applications. Creatingmembrane structures having fine pores and high flux is difficult, as theflux through the membrane decreases with decreasing pore size, arelationship that urges the use of layers that are as thin aspracticable. Fabricating thin porous layers with uniform pores overlarge surface area and that are mechanically robust is a challengingtask. Therefore, thin, fine porous membranes typically are stacked onthicker substrates with coarser pores. In such membrane structures it isextremely difficult to control the layer thickness of the fine porelayer to within a few microns. In spite of much effort, the currentlyavailable membrane structures with fine pores exhibit undesirably lowpermeance. Therefore, it is desirable to improve the efficiency of fineporous membrane structures suitable for high temperature, high pressure,and/or corrosive atmospheres and to develop suitable methods tofabricate such structures.

SUMMARY OF THE INVENTION

The present invention meets these and other needs by providing amembrane structure having high flux and high selectivity. Accordingly,one embodiment of the invention is a membrane structure. The membranestructure includes a first layer having a plurality of pores; and asecond layer disposed on the first layer. The second layer has aplurality of unconnected pores, wherein at least a portion of theplurality of unconnected pores is at least partially filled with afiller. At least a portion of the plurality of unconnected pores is influid communication with at least one of the pores of the first layer.The first layer is substantially free of the filler.

A second embodiment of the invention is a method of making a membranestructure. The method includes the steps of providing a first layerhaving a plurality of pores; disposing a second layer on the firstlayer, wherein the second layer comprises a plurality of unconnectedpores; and filling at least a portion of the plurality of unconnectedpores at least partially with a filler such that the first layer issubstantially free of the filler. Disposing a second layer includesdepositing a metal layer on the first layer; and anodizing the metallayer to convert the metal layer into porous oxide layer.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic representation of a membrane structure, accordingto one embodiment of the present invention;

FIG. 2 is a schematic representation of a gas separation assemblyincorporating membrane structure of the invention, according to oneembodiment of the invention;

FIG. 3 is a schematic representation of a filter incorporating membranestructure of the invention, according to one embodiment of theinvention;

FIG. 4 is a flow chart of a method of making membrane structure,according to one embodiment of invention; and

FIG. 5 is a schematic representation of a method of making membranestructure, according to one embodiment of invention.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top,” “bottom,”“outward,” “inward,” “first,” “second,” and the like are words ofconvenience and are not to be construed as limiting terms. Furthermore,whenever a particular aspect of the invention is said to comprise orconsist of at least one of a number of elements of a group andcombinations thereof, it is understood that the aspect may comprise orconsist of any of the elements of the group, either individually or incombination with any of the other elements of that group.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing one embodiment of theinvention and are not intended to limit the invention thereto.

For the purposes of understanding the invention, the term “layer havinga plurality of unconnected pores” is to be understood to be a porouslayer containing a substantial number of substantially isolated pores,such as parallel and unconnected channels. In other words, the pores aredisposed such that there is only incidental internal fluid communicationamong the pores. Of course, one skilled in the art will recognize thatan occasional defect is to be expected in fabricating such structures,and so a structure containing occasional defects, such as pore channelbranches, will still be considered a structure having a plurality ofunconnected pores, if the number of defects is not sufficient tosubstantially alter the performance of the structure relative to whatwould be expected for a defect-free structure.

A schematic representation of a membrane structure according to oneembodiment of the present invention is shown in FIG. 1. The membranestructure 10 of FIG. 1 includes a first layer 12 comprising a pluralityof pores 13, and a second layer 14 having a plurality of unconnectedpores 16; second layer 14 is disposed on the first layer 12. At leastsome of the unconnected pores 16 of the second layer 14 are at leastpartially filled with a filler 18 such that the first layer 12 issubstantially free of the filler 18. In such membrane structures thesecond layer 14 is an “active” layer of the membrane 10, that is, thislayer provides desired selectivity and/or functionality, and the firstlayer 12 provides the desired mechanical stability and support. At leasta portion of the plurality of the pores 13 of the first layer 12 is influid communication with at least one of the pores 16 of the secondlayer 14. Typically, a substantial fraction, for example, at least about75%, of the unconnected pores of the second layer 14 is in fluidcommunication with at least one of the pores (13) of the first layer 112to sustain a desirable flux through the membrane structure 10.

Typically first layer 12 includes a layer with a plurality ofinterconnected or unconnected pores; its primary purpose is to providemechanical strength to structure 10 and to support high flux of fluid.The first layer 12 may be a porous ceramic, a porous metal or a porouspolymer layer. When the first layer 12 comprises a metal layer, themetal may be passivated with a polymer or a ceramic layer. In oneembodiment, the first layer 12 has a porosity volume fraction of atleast about 1%. In another embodiment, the first layer 12 has a porosityvolume fraction in the range from about 20% to about 70%. In yet anotherembodiment, the first layer 12 has a porosity volume fraction in therange from about 30% to about 50%.

The total thickness of the membrane structure 10 is chosen in such a waythat the structure is thick enough for mechanical robustness, but not sothick as to impair permeability. The thickness of the individual layersis optimized depending on the end use application. As mentioned above,the need for high flux drives the need for fine porous layers to be asthin as possible. In one embodiment, the second layer 14 has a thicknessless than about 10 micrometers. In certain embodiments, the second 14layer has a thickness in the range from about 10 nanometers to about 1micrometer. In particular embodiments, the second layer 14 has athickness in the range from about 10 nanometers to about 200 nanometers.As will be described in more detail, below, the methods used for thefabrication of the membrane enable very good control over the thicknessand pore structure of the layers.

Precise control over pore size and pore size distribution, especiallythose of the second layer 14, are among the parameters that define theoverall membrane structure performance. The pore size of the layers ischosen based on the end use application of the membrane structure 10. Insome embodiments, the second layer 14 has a median pore size of lessthan about 1 micrometer. In certain embodiments, the median pore size ofthe second layer 14 is in a range from about 1 nanometer to about 500nanometers. In particular embodiments, the median pore size of thesecond layer 14 is in the range from about 1 nanometer to about 100nanometers.

The plurality of pores in the second layer 14 has at least one porearchitecture. A “pore architecture” is a plurality of parallel poreshaving a particular average size and morphology. Typically, for eachpore architecture, the pore size distribution does not vary considerablycompared to the value of the median pore size of that architecture.However, some variation is tolerable as long as the performance of themembrane is not adversely affected. In some embodiments, the pluralityof pores may comprise at least two pore architectures; in such cases theplurality of pores may have a plurality of size ranges. In all the aboveembodiments, the pore size of the first layer 12 is chosen so that thepores do not unduly hinder the permeance of the species through themembrane structure. In an exemplary embodiment, the second layer 14includes a plurality of cylindrical pores of uniform size, withsubstantially all pores aligned approximately perpendicular to themembrane surface.

At least some of the pores of the second layer 14 are at least partiallyfilled with a filler 18. In certain embodiments, at least about 50% ofthe pore volume is filled with the filler 18, in other embodiments, atleast about 75% of the pore volume is filled with the filler 18. Incertain embodiments, the filler 18 completely fills at least some of theunconnected pores 16. The first layer 12 is substantially free of thefiller 18. In certain embodiments, the filler 18 comprises a porousmaterial. Alternatively, the filler 18 may be a dense material. Inembodiments where the filler 18 is porous, the porous filler has amedian pore diameter of less than about 50 nanometers. In certainembodiments, the porous filler has a median pore diameter in the rangefrom about 0.5 nanometer to about 20 nanometers. In certain embodimentsthe filler disposed in at least one pore of the second layer comprises adifferent material from the filler disposed in another pore of thesecond layer. In certain embodiments, a pore may be filled with morethan one kind of filler material. For example, a pore may be filledpartly with one filler material and partly with another filler.Moreover, the filler may comprise more than one chemical species, suchas, for instance, a mixture of more than one type of oxide, or a mixtureof metal with metal oxide. In certain embodiments, the pore walls of thefiller porous material may be functionalized with a functional material,as will be discussed in detail below.

The filler 18 typically comprises a ceramic, a metal, or an organicmaterial. Examples of suitable ceramic materials include, but are notlimited to, oxides, borates, aluminates, silicates, and phosphates,individually or in any combination thereof. In one embodiment, thefiller material comprises an oxide. Examples of suitable oxides include,but are not limited to, oxides of silicon (Si), titanium (Ti), aluminum(Al), zirconium (Zr), niobium (Nb), tantalum (Ta), tungsten (W), tin(Sn), hafnium (Hf), iron (Fe), cerium (Ce) and yttrium (Y) in theirstoichiometric or non-stoichiometric forms, either individually or inany combination thereof. In certain embodiments, the filler may comprisea complex oxide of more than one metal of the form ABO_(x), where Aincludes, but is not limited to, Mg, Ca, Ba, and Sr, and B includes, butis not limited to, Zr, Ti, Si, and Al. The filler may also comprisedoped oxides such as yttria stabilized zirconia, and the like. In aparticular embodiment, the oxide comprises silica (SiO₂). In anotherparticular embodiment, the oxide comprises titania (TiO₂). In oneembodiment, the filler material comprises a plurality of compositions.The plurality of compositions may comprise, for example, any combinationof the oxides listed above. Furthermore, the composition may be dopedwith any desired dopant.

In certain embodiments, the filler material comprises a metal. In oneembodiment, the filler material comprises a transition metal. Inparticular embodiments, the filler material comprises a platinum groupmetal, iron, nickel, cobalt, copper, or combinations thereof. In oneembodiment, the metal comprises palladium. Palladium has high permeancefor hydrogen and is advantageous for hydrogen separation applications.In certain embodiments, the filler material comprises alloys of theabove listed metals.

In certain embodiments, the filler material comprises an organicmaterial. In certain embodiments, the organic material comprises apolymer. Examples of suitable polymers include, but are not limited to,a polysulfone, a polyamide, a cross-linked polyimide, a polyetherketone, a polyetherimide, a silicone rubber, a nitrile rubber, aneoprene rubber, a silicone, a polycarbonate, a polyarylene, apolyphenylene ether, a polyolefin elastomer, a polybutadiene, apoly-ionomer, a polyionic liquid, a polyethylene oxide, a polypropyleneoxide, a vinyl polymer, a polynorbomene, a cellulose acetate, apolydimethylsiloxane, a polyvinylidene fluoride, a polynorbomene, andcombinations thereof. In one embodiment, the polymer comprises acopolymer. In certain embodiments the copolymer may be a blockcopolymer. In certain embodiments, the polymer comprises a liquidpolymer.

The filler material may further comprise a plurality of nanoparticles,each having a diameter that is less than the size of the unconnectedpores 16 of the second layer 14. In one example, the plurality ofnanoparticles comprises semiconductor nanocrystals. In another example,the plurality of nanoparticles comprises a ceramic material, asdescribed above.

The filler material provides a wide variety of candidate materials andsurface characteristics, which may be useful for functionalization andselective adsorption. The porous filler material advantageously providespore size ranges smaller than those of the unconnected pores. By fillingthe pores of the second layer 14 with a porous filler material, it ispossible to achieve pores with controlled diameters and pore morphology.

Though typically, the filler is disposed in the pores of the secondlayer 14, and the first layer 12 is substantially free of the filler 18,in certain embodiments the first layer 12 may have a second fillermaterial. The second filler material, in some embodiments, comprises amoisture sorbent, or a catalyst. Such structures may be advantageous forpre-treatment, such as moisture removal, to be performed on the fluidpassing through structure 10 prior to passing through the active secondlayer 14.

The materials of the first layer 12 and the second layer 14 are chosenbased on the end use application. Typically the first layer 12 includeseither a polymer or a ceramic with suitable porosity, pore dimensions,and thickness. In an exemplary embodiment, the first layer 12 includes aceramic. Non-limiting examples of ceramics are an oxide, a carbide, anitride, a boride, and a silicide. Examples of suitable ceramicsinclude, but are not limited to, aluminum oxide, silica, silicate,rare-earth oxide, titania, zirconia, lanthana, yttria stabilizedzirconia, a perovskite, a spinel, vanadia, ceria, and combinationsthereof. In some embodiments, the ceramic may include a suitable dopant.Ceramic materials have the advantages of thermal and chemical stability,good erosion resistance, and high-pressure stability. Thus the membranestructures of the embodiments may withstand prolonged exposure topressure or temperature differences that may be present in, for example,a gas separation or sensor assembly.

In some embodiments, the first layer 12 includes a polymer. Examples ofsuitable polymers include, but are not limited to, a polysulfone, apolyamide, a cross-linked polyimide, a polyether ketone, apolyetherimide, a silicone rubber, a nitrile rubber, a neoprene rubber,a silicone, a polycarbonate, a polyarylene, a polyphenylene ether, apolyolefin elastomer, a polybutadiene, a poly-ionomer, a polyionicliquid, a polyethylene oxide, a polypropylene oxide, a vinyl polymer, apolynorbomene, a cellulose acetate, a polydimethylsiloxane, apolyvinylidene fluoride, and various combinations thereof. In oneembodiment, the polymer comprises a copolymer. In certain embodimentsthe copolymer may be a block copolymer. These polymers may be used toachieve specific functionalities. For example, silicone rubber is veryeffective in removing volatile organic components such as toluene,methanol, methylene chloride, and acetone from gas streams.

In certain embodiments, the first layer 12 includes more than onesublayer. In such embodiments, a sublayer not in contact with the secondlayer may include an unpassivated metal. A pure metal or a metal alloymay be used. The metal may be applied on the membrane layers as adispersed particulate, or a continuous coating, or a metal layer may beinserted into the membrane structure. In some embodiments, the porewalls of first layer 12, the second layer 14, or the porous filler maybe coated with a metal. The metal may be disposed into the membranestructure 10 by any known coating technique, including exposing thestructure to a suspension of metal particulates; electroless deposition;electroplating; chemical vapor deposition; or physical vapor depositiontechniques. In some embodiments, the metal is a platinum group metal. Inone embodiment, the metal comprises palladium, which, as mentionedpreviously, may be advantageous for hydrogen separation applications. Inone embodiment palladium with copper, gold or silver is used. In anotherembodiment, an alloy of palladium with ruthenium, osmium, nickel,platinum, or a combination of these is used. In some embodiments, thetransition metal elements such as iron, nickel, cobalt, or copper may beincluded in the membrane structure. Many transition metal complexes showselective interaction with molecular oxygen involving reversiblechemisorption, and thus may be suitable for oxygen separation. Thesecomplexes may include a transition metal ion and a polydentate ligand.Some examples of suitable complexes are Co or Ni or Cu embedded inpolyphyrins or oximes, to which axial bases such as nitrogen or sulphurare attached. Selection and production of these complexes are known tothose skilled in the art.

Typically, the second layer 14 includes an oxide product of ananodization process. Some examples of such oxides include, but are notlimited to, alumina, titania, silica, tin oxide, zirconia, niobiumoxide, tungsten oxide, molybdenum oxide, tantalum oxide, analuminosilicate, or combinations of one or more of these. In someembodiments, the second layer 14 may include oxides of metals comprisingaluminum, titanium, tin, zirconium, niobium, tungsten, molybdenum, ortantalum. In an exemplary embodiment, the second layer 14 comprisesalumina. Such oxides have the advantages of thermal and chemicalstability, good erosion resistance, and high-pressure stability.

In certain embodiments, the second layer 14 comprises more than onesublayer. In certain embodiments, at least one sublayer in the pluralityof sublayers comprises a different value than another sublayer in theplurality, for at least one parameter such as a median pore size,sublayer thickness, and the like, depending on the requirement of theend use application. Each of the sublayers comprises a plurality ofunconnected pores, and the porous structure of these sublayers, likethat of the second layer in general, is in fluid communication with oneor more pores of the first layer 12, to allow fluid flow through themembrane structure 10.

In one embodiment, the sublayers of the second layer 14 may have amonotonic variation in pore dimension across the layer thickness; thatis, the pore diameters vary systematically across the layer thickness.For example, the pore diameter may increase or decrease systematicallyacross the height of the layer; In certain embodiments, the sublayerexposed to the surface has a finer pore size than a sublayer disposedbeneath it. Alternatively, in another embodiment, the sublayer exposedto the surface has a coarser pore size than a sublayer disposed beneathit. In another embodiment; the sublayers may have a nonmonotonicvariation in pore dimension across the membrane. The thickness and poredimensions of each of the layers are chosen depending on the end useapplication.

By tuning the pore dimensions, the properties of the membrane structure10 may be controlled to provide performance suitable for any of a numberof applications. For example, such membrane structures may be utilizedas high flux membranes with Knudsen selectivity for gases. If the gasesdo not interact with the membrane surface, membranes prepared using themethod described above could be used to separate gases using a Knudsenmechanism. Membrane-structures in accordance with this embodiment of thepresent invention may allow higher fluxes than for membranes withthicker active layers.

In some embodiments, at least one of the layers includes a catalyticmaterial. For example, a catalytic coating or a catalytic layer disposedwithin the membrane structure may enable structure 10 to combinemembrane separation with catalytic reaction to achieve high efficiencyfluid mixture separation. The catalyzed reaction may be used, forinstance, to reduce the concentration of one or more of the reactionproducts within the membrane structure, hence increasing the conversionefficiency. Catalytic materials may also be included in the membranestructure for microreactor or sensor applications. Some examples ofcatalysts include, but are not limited to, platinum, palladium, copper,copper oxide, ceria, zinc oxide, alumina, combinations thereof, oralloys thereof.

One skilled in the art would know how to choose a catalyst materialbased on the desired reaction and given working environment, thendispose the desired catalyst into the membrane structure. The catalystsmay be disposed onto the structure by a number of coating techniques.They may be deposited by a physical vapor deposition or by chemicalmeans. Examples of physical vapor deposition include, but are notlimited to, evaporation, e-beam deposition, ion beam deposition, or asuitable combination of these techniques. The catalyst may also bedisposed into the membrane structure by means of chemical vapordeposition, including atomic layer deposition. The pores of the membranestructure may also be filled with a catalyst by simple capillaryfilling, or by spray coating. In such embodiments, the catalyst to bedisposed may be taken as a sol, a solution or a gel. In someembodiments, the pore walls of one or more layers are coated with acatalyst. Alternatively, in some other embodiments, a catalyst layer maybe disposed within the membrane structure.

The membrane layers may be functionalized with a suitable functionalgroup to achieve specific functional properties. The functional group,in some embodiments, may be an acid, a basic, an amine, a hydroxyl, acarbonyl, a carboxyl, a mercapto group, a vinyl group, an alkyl, afluoroalkyl, a benzyl, or an acryl group. These functional groups alterthe surface properties of the membrane materials and impart specificproperties to the membranes. For example, the functional groups may beused to change the wettability of the membrane pore surfaces to controlthe flow of fluid through the membrane.

Functionalizing the pore surfaces is especially useful for biological orbiomedical applications where the membranes desirably be hydrophilic,hydrophobic, lyophobic or lyophilic. The functional groups may be usedto control the flow of specific chemical or biological species throughthe membrane. Specific functional groups may be used to control theattachment of cells or proteins to the membrane structure. For example,the functional groups may also be used to make the membrane structurebiocompatible for biomedical applications. The functional groups may bedisposed onto the membrane structure by any known coating technique. Insome embodiments, the functional group may be attached to the selectedregions of the layers by exposing the layers to solutions or vapor orions including the desired species. Pretreatment of the layers toenhance the adhesion of the functional groups and masking of regions tobe protected during coating may be required. Techniques for carrying outsuch a pretreatment are known to those skilled in the art.

In some embodiments, the membrane structure includes a compositematerial. The composite may include a ceramic-organic or aceramic-ceramic composite. Any ceramic including those listed above maybe used in the composite. The organic material may include a polymer, anoligomer, or a monomer.

The membrane structure 10 of the embodiments may be useful in a numberof applications. In some embodiments, the membrane structure 10 is partof a separation assembly. The membrane structure 10 in certainembodiments of the invention may be capable of molecular sievingsuitable for purification of natural gas, or may be applied forseparation of various species from a fluid flow, including suchapplications as air separation, NO_(x) separation, oxygen separation,and hydrogen recovery from processing gases or feedstock. In oneembodiment, the membrane structure 10 of the embodiments may be used forseparation of hydrogen from nitrogen, argon, carbon dioxide, or methane.In another embodiment, the membrane structure 10 of the invention may beused for separation of volatile organic components from air streams. Insome embodiments the membrane structure 10 is a part of a hightemperature gas separation unit. For such applications, a suitable metalor a polymer coating may be applied on one or more layers of themembrane structure. Alternatively, a metal or a polymer layer may beused in conjunction with the membrane structure.

FIG. 2 shows a schematic representation of a simple gas separation unit30 according to one embodiment of the invention. The unit 30 includes acompressor 32, a coalescing filter 33 and a pre-heater unit 34 connectedto a membrane separation unit 36. Air under pressure flows first throughthe coalescing filter 33 and then through the pre-heater unit 34 beforereaching the membrane separation unit 36. The coalescing filter may beused to remove oil or water droplets or particulate solids from thefeed. The membrane separation unit includes one or more of membranestructure of the invention configured to remove a desired component fromthe air mixture. The desired component passes through outlet 37, leavingthe waste permeate gases through outlet 38. The membrane separation unitmay include additional heaters or additional filters.

The membrane structure 10 may be used as a liquid-liquid separationassembly such as separation of water from fluid containing organiccomponents. For such applications, the membrane structure 10 may becombined with other porous or non-porous separation layers if needed. Inone embodiment, a separation layer of non-porous cross-linked polyvinylalcohol layer of suitable thickness is used in conjunction with themembrane structure. The pore structure and thickness of each of thelayers may be adjusted depending on the requirement. In someembodiments, the membrane structure 10 may be a membrane structure in aseparation assembly that also includes a reactor component coated on thepore walls to prevent fouling.

In one embodiment, the membrane structure 10 is part of a filtrationassembly. By controlling the pore dimensions of the layers, the membranestructure 10 of the invention may be used for microfiltration to filterout solid particles with dimensions less than about 10 micrometers, orfor ultrafiltration to filter out particles with dimensions down toabout 50 nanometers such as separation of macromolecules and bacteria.By choosing the pore dimensions of the layers to very small sizes, it ispossible to use these membrane structures for hyperfiltration to filterout still smaller units such as sugars, monomers, aminoacids, ordissolved ions by reverse osmosis. In one embodiment, the membranestructure is a part of a bio-separation or reaction assembly. The poresize and thickness of the membrane layers are chosen depending on thesizes of the species to be separated. Accordingly in one embodiment, themembrane structure 10 is a filter usable in food, pharmaceutical, andindustrial applications. In another embodiment, the membrane structure10 is a part of a protein purification unit.

FIG. 3 shows a schematic representation of a simple filter unit 40according to one embodiment of the invention. The unit 40 includes afeed tank 42 used for storing the liquid medium containing the materialto be separated. The circulation of the feed 43 is controlled by thepump 44 that draws the feed 43 through lines 46 and 48 into a membranefilter assembly 50. The membrane filter assembly 50 includes one or moreof the membrane structure of the invention configured to filter out aspecific component from the feed. The desired component ‘filtrate’ 47passes through outlet 49, while the retentate 52 may be removed orreturned to the feed tank 42.

In one embodiment, the membrane structure 10 is part of a reactorassembly, performing similar functions to conventional membranes presentin reactors such as filtration and separation. In another embodiment,the membrane structure 10 is capable of reactive separation wherein themembrane structure 10 is a reactor that also separates one of theproducts. In an exemplary embodiment, the membrane structure 10 is apart of a chemical microreactor assembly that generates hydrogen fuelfrom liquid sources such as ammonia. In such embodiments, suitablehydrogen permselective catalysts are used in the membrane structure.

In one embodiment, the membrane structure 10 is part of a sensorassembly. In such embodiments, the membrane layers may be functionalizedwith functional groups as discussed above, to incorporate reversiblechanges within the membrane structure. Examples of reversible changesinclude, but not limited to, chemical reactions such as ionization,oxidation, reduction, hydrogen bonding, metal complexation,isomerization, and covalent bonding. These changes may be utilized todetect a chemical or a biological species, or to detect change intemperature, pH, ionic strength, electrical potential, light intensityor light wavelength. The use of membrane structures for sensorapplications is expected to enhance the performance of detection becauseof their high surface to volume ratio.

Another aspect of the invention is to provide a method for preparing amembrane structure. A flow diagram of the method of making a membranestructure is shown in FIG. 4. The method 60 begins with step 62, whereina porous first layer is provided. In step 64, an electrically conductingcoating with an outer and an inner surface is deposited on the porousfirst layer. As used herein, the inner surface is adjacent to the firstlayer and the outer surface is the surface away from the first layer. Instep 66, the electrically conducting layer is anodized in an acidelectrolyte to form a porous second layer starting from the outersurface, such that a thin oxide barrier layer is present at theinterface between the first layer and the second (anodized) layer. Theformation of the barrier layer is a byproduct of the anodizationprocess. In step 67, at least a portion of the pores of the second layeris at least partially filled with a filler material. The first layer issubstantially free of the filler because the barrier layer restricts thefiller to the second layer. After filling the desired portion of thepores, the barrier layer is removed by etching in step 68 to obtain amembrane structure.

The schematic of the process steps of a method 70 according to oneembodiment of the invention is shown in FIG. 5. To begin with, a porousfirst layer 72 is provided. Any fabrication technique suitable forfabricating porous layers may be used to fabricate the first layer. Inembodiments where the first layer includes a ceramic, the layer may bemade by a casting process. To start with, a slurry including the ceramicpowder of the desired material is prepared. The slurry may include abinder and a curing agent. The amount of powder in the slurry isgenerally adjusted to have the rheological properties suitable forcasting. Further additive agents may be mixed into the slurry, such as adispersing agent for improving the dispersibility and to prevent rapidsettling, and a plasticizer for improving the binding force between thebinder and the ceramic particles and to lower the risk of cracking.Typically a layer is formed on a substrate by applying the slurry on thesubstrate. Any technique known in the art for preparing layers may beused for forming the first layer. Non-limiting examples of usefulformation techniques include, but are not limited to, spraying, screenprinting, ink-jet printing, casting, wire-bar coating, extrusioncoating, gravure coating, roll coating, and combinations thereof. Insome exemplary embodiments, a casting technique, such as tape casting,is used. Tape casting proves useful for making large area thin ceramicsheets with controlled thickness and porosity. The process may includean intermediate a curing process to remove organic binders and solventsand a sintering step to densify the layer. Exemplary sinteringtechniques may involve heating at a specified temperature for aspecified duration, or microwave irradiation, or electron beamirradiation, or UV light exposure, or a combination of those. Theporosity, pore size, and pore size distribution is controlled by theparticle sizes of starting material. In some cases, it is desirable tostart with particles of uniform particle sizes in order to achieveuniform pore structure with minimal pore size distribution. Defect freelayers with desired porosity may be obtained by a precise control ofsintering conditions.

In embodiments where the first layer includes a polymer, any techniqueknown in the art to make porous polymer layers may be used to fabricatethe first layer. For example, a thermoplastic, or a thermoplasticelastomer, or a thermoset polymer may be mixed with a porogen and castinto a thin layer of desired thickness. The layer may be heat treated orexposed to light or any other radiation to convert the layer into aporous layer. When the polymer chosen is a blend of two polymers, it ispossible to control the casting conditions, such as the selection ofsolvents used, to get a porous layer by phase separation.

In step 64, a coating of electrically conducting material 73 having anouter surface 74 and an inner surface 75 is disposed on the first layer72. The inner surface 75 is adjacent to the first layer 72. Thethickness of the second layer is determined by the thickness of thedeposited conducting coating. Any coating technique known in the art maybe used for depositing the conducting layer. Some examples of suitablecoating techniques include, but are not limited to, physical vapordeposition, chemical vapor deposition, and electroless depositiontechnique.

In step 66, the electrically conducting coating 73 is anodized in anacid electrolyte such that a porous layer 76 is formed starting from theouter surface 74, such that a thin barrier layer 78 is present at theinterface between the first layer and the porous second layer 76. Theformation of the barrier layer (78) is a byproduct of the anodizationprocess. It is known that certain materials such as aluminium, silicon,tin, titanium, zirconium, niobium, tungsten, molybdenum, tantalum, andtheir alloys form a porous oxide layer when anodized in an acid medium.The simultaneous formation of oxide layer at the conducting layersurface and dissolution of the formed oxide into the acid give rise to apeculiar porous structure including a plurality of cylindrical pores ofuniform size. Typically, a strong acid such as a phosphoric, a sulfuric,or an oxalic acid is used as an electrolyte. The pore size and thespacing between the pores may be controlled by adjusting the voltageduring anodic oxidation. The thickness of the oxide film formed iscontrolled by the thickness of the metal film. Anodization terminateswhen all the metal is consumed and converted to oxide, leaving a thinoxide barrier between the first and the second layers. Thus a secondlayer 76 with a plurality of unconnected pores 80 of controlled poredimension is obtained.

In step 68, the pores 80 of the second layer 76) are filled with filler82. The filler material may be introduced into the pores of the secondlayer 76) by any known technique, from the top surface of second layer76). Examples of, suitable filling processes include, but are notlimited to, spin casting, injection, spray coating, pressureinfiltration, electrophoretic deposition, electrodeposition, andcapillary filling of the filler material. The exact process used dependson the nature of the filler material, desired structure of the membrane,cost, and various other criteria.

When the filler material is a porous ceramic, the filler material may beprovided in a liquid precursor form. At least one liquid precursor of atleast one porous ceramic filler may be provided in a solvent. Thesolvent may comprise water, methanol, propanol, butanol, or combinationsthereof. The solvent mixture may include other less polar solvents tomodify or adapt polarity; such solvents include, among others, aceticacid, formic acid, formamide, acetone, methylethylketone, ethylacetate,acetonitrile, N,N-dimethyl formamide, and dimethyl sulfoxide, or anycombination thereof. The liquid precursor is provided in an amountsufficient to fill a predetermined number of the pores of the secondlayer to a predetermined portion(s). In certain embodiments, the liquidprecursor comprises at least one template and at least one ceramicprecursor. The template provides the organization and size range of thepore architecture. Examples of the template include a cationicsurfactant, a non-ionic block copolymer, a protein, an anionicsurfactant, a nonionic surfactant, or any combinations thereof. Examplesof the ceramic precursor include alkoxides, metal salts, oxide colloidalparticles, or any combinations thereof. The liquid precursor mayoptionally comprise at least one reagent. Examples of a reagent include,but are not limited to, an acid, a base, and a salt, either individuallyor in any combination. Examples of an acid include, but are not limitedto, hydrochloric acid, nitric acid, sulfuric acid, acetic acid, carbonicacid, and citric acid. Examples of a base include, but are not limitedto, ammonium hydroxide, sodium hydroxide, and tetramethylammoniumhydroxide. Examples of a salt include, but are not limited to, sodiumchloride, potassium chloride, sodium acetate, sodium fluoride, andethylenediaminetetraacetic acid tetrasodium. Furthermore, the liquidprecursor may be doped with a desired dopant.

Typically, during any wet chemical method of filling the pores such asdip coating, the bottom surface of the first layer is covered with amask such that the liquid precursor does not enter the pores of thefirst layer. The barrier layer 78 present between the first layer 72 andthe second layer 76 prevents the filler material from entering the firstlayer 72. After filling the desired portion of the pores 80 of thesecond layer 76 with a filler, a portion of the solvent from the liquidprecursor is removed to form the liquid precursor into a gel. As anexample, the solvent may be removed by evaporation, using temperatureassisted or vacuum-assisted methods, or a combination of the twomethods. Subsequently, the barrier layer 78 between the porous oxidelayer 76 and the first layer 72 may be removed by chemical etching, forexample in a dilute acid or base or in a reducing environment, to obtaina membrane structure 84.

By using different templates or ceramic precursors in each fillingstage, adjacent regions with different pore size and or poreorganization may be sequentially deposited. Furthermore, the method alsoprovides independent control over the relative size and region of a porearchitecture through the order of filling and the extent of shrinkageduring the heating step.

It is possible to fabricate membrane structures with differentconfigurations by tuning the anodization conditions. For example, amembrane with more than one sublayer of second layer may be fabricatedby changing periodically the anodization parameters. Once the desiredthickness of the sublayer is formed, the structure is, for example,anodized at a reduced anodization voltage to obtain a sublayer withfiner pores. Using this exemplary technique it is possible to introduceany number of sublayers with sequentially reducing pore dimensions tofabricate asymmetric membranes by tuning the anodization current orvoltage.

Anodization of a conducting layer to obtain the active layer of themembrane provides various advantages. This technique enables goodcontrol over the thickness of the active layer. Additionally, throughthe use of the techniques described above, the filler is mainly confinedto the pores of the active layer having finer pores and hence theincorporation of the filler into the membrane structure does notadversely affect the flux. Membrane structures of the prior art thatincorporate filler materials into porous membranes generally includefiller throughout the entire membrane structure and hence the fluxthrough such membrane structures may be undesirably reduced. Theintegrated membrane structure described herein is superior to suchstructures due to their good mechanical adhesion between the layers,high flux, and the good fluid communication between the pores of theadjacent layers.

The following example serves to illustrate the features and advantagesoffered by certain embodiments of the present invention, and is notintended to limit the invention thereto:

EXAMPLE

The following example describes the preparation method for makinganodized alumina membrane structures.

A 1-2 μm thick evaporated aluminum film is deposited onto the topsurface of a porous alumina substrate. The aluminum film is anodized inoxalic acid (0.3 M). The anodization voltage of between 30 V and 80 V ischosen based on the pore size and spacing requirements. Once the film isfully anodized, the pores in the anodized alumina are separated from theporous support by a continuous alumina barrier layer, a byproduct of theanodization process. The bottom surface of the porous support is maskedwith paraffin and the sample is dip-coated in a mesoporous silicaprecursor. The oxide barrier layer prevents the precursor solution frompermeating the porous support. The mask prevents permeation into theporous support from the bottom surface of the porous support. Mesoporousoxide is formed by calcining the coated structure. The top surface ofthe structure is now masked off and then dipped in 5 wt % phosphoricacid, which etches away the barrier layer through the support, withoutcompromising the filler material.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled, in the art. It is, therefore, to be understood that theappended claims are intended to cover all such modifications and changesas fall within the true spirit of the invention.

1. A membrane structure comprising: a first layer comprising a pluralityof pores; and a second layer disposed on the first layer, the secondlayer having a plurality of unconnected pores; wherein at least aportion of the plurality of unconnected pores are at least partiallyfilled with a filler; wherein at least a portion of the plurality ofunconnected pores is in fluid communication with at least one of thepores of the first layer; and wherein the first layer is substantiallyfree of the filler.
 2. The membrane structure of claim 1, wherein thefirst layer comprises a material selected from the group consisting of aceramic, a metal, and a polymer.
 3. The membrane structure of claim 1,wherein the first layer has a porosity volume fraction greater thanabout 1%.
 4. The membrane structure of claim 3, wherein the first layerhas a porosity volume fraction in a range from about 20% to about 70%.5. The membrane structure of claim 4, wherein the first layer has aporosity volume fraction in a range from about 30% to about 50%.
 6. Themembrane structure of claim 1, wherein the second layer comprises anoxide of a material selected from the group consisting of silicon (Si),titanium (Ti), aluminum (Al), zirconium (Zr), niobium (Nb), tantalum(Ta), tungsten (W), tin (Sn), hafnium (Hf), iron (Fe), yttrium (Y),combinations thereof, and alloys thereof.
 7. The membrane structure ofclaim 1, wherein the second layer comprises a material selected from thegroup consisting of alumina, aluminosilicate, and titania.
 8. Themembrane structure of claim 6, wherein the second layer comprisesalumina.
 9. The membrane structure of claim 1, wherein the second layerhas a thickness less than about 10 micrometers.
 10. The membranestructure of claim 1, wherein the second layer has a thickness in therange from about 10 nanometers to about 1 micron.
 11. The membranestructure of claim 10, wherein the second layer has a thickness in therange from about 10 nanometers to about 200 nanometers.
 12. The membranestructure of claim 1, wherein the second layer has a median pore size ofless than about 1 micrometer.
 13. The membrane structure of claim 1,wherein the second layer has a median pore size in the range from about1 nanometer to about 500 nanometers.
 14. The membrane structure of claim13, wherein the second layer has a median pore size in the range fromabout 1 nanometer to about 100 nanometers.
 15. The membrane structure ofclaim 1, wherein the second layer comprises a plurality of sublayers.16. The membrane structure of claim 15, wherein at least one sublayer inthe plurality of sublayers comprises a different value than anothersublayer in the plurality, for at least one parameter selected from thegroup consisting of median pore size and sublayer thickness.
 17. Themembrane structure of claim 1, wherein the filler comprises a porousfiller.
 18. The membrane structure of claim 17, wherein the porousfiller has a median pore diameter of less than about 50 nm.
 19. Themembrane structure of claim 17, wherein the porous filler has a medianpore diameter in the range from about 0.5 nanometer to about 20nanometers.
 20. The membrane structure of claim 1, wherein the fillerdisposed in at least one pore of the second layer comprises a differentmaterial from the filler disposed in another pore of the second layer.21. The membrane structure of claim 1, wherein the filler comprises amaterial selected from the group consisting of a ceramic, a metal, anorganic material, and combinations thereof.
 22. The membrane structureof claim 21, wherein the ceramic comprises an oxide of a metal selectedfrom the group consisting of silicon, aluminum, titanium, zirconium,hafnium, cerium, iron, tantalum, tungsten, niobium, yttrium, tin, andcombinations thereof.
 23. The membrane structure of claim 21, whereinthe ceramic comprises an oxide of the form ABO_(x).
 24. The membranestructure of claim 23, wherein A comprises a material selected from thegroup consisting of Mg, Ca, Ba, and Sr; and wherein B comprises amaterial selected from the group consisting of Zr, Ti, Si, and Al. 25.The membrane structure of claim 21, wherein the metal comprises atransition metal.
 26. The membrane structure of claim 21, wherein themetal comprises at least one selected from the group consisting of aplatinum group metal, iron, nickel, cobalt, copper, combinationsthereof, and alloys thereof.
 27. The membrane structure of claim 21,wherein the organic material comprises a polymer.
 28. The membranestructure of claim 27, wherein the polymer comprises one selected fromthe group consisting of a polyethersulfone, a polyamide, a cross-linkedpolyimide, a polyether ketone, a polyetherimide, a silicone rubber, anitrile rubber, a neoprene rubber, a silicone, a polycarbonate, apolyarylene, a polyphenylene ether, a polyolefin elastomer, apolybutadiene, a vinyl polymer, a poly-ionomer, a polyionic liquid, apolyethylene oxide, a polypropylene oxide, a cellulose acetate, apolydimethylsiloxane, a polyvinylidene fluoride, a polynorbomene,combinations thereof, and copolymers thereof.
 29. The membrane structureof claim 1, wherein at least one of the layers comprises a catalyticmaterial.
 30. The membrane structure of claim 1, wherein at least one ofthe layers comprises a functional group.
 31. The structure of claim 30,wherein the functional group is at least one selected from the groupconsisting of an amine group, a carboxyl group, a mercapto group, acarbonyl group, a hydroxyl group, a vinyl group, an alkyl group, abenzyl group, a fluoroalkyl group, and an acryl group.
 32. The membranestructure of claim 1, wherein at least a portion of the plurality ofpores of the first layer comprises a second filler.
 33. The membranestructure of claim 32, wherein the second filler comprises a moisturesorbent or a catalyst.
 34. A separation assembly comprising the membranestructure of claim
 1. 35. A filtration assembly comprising the membranestructure of claim
 1. 36. A reactor assembly comprising the membranestructure of claim
 1. 37. A sensor assembly comprising the membranestructure of claim
 1. 38. A membrane structure comprising: a first layercomprising a plurality of pores; and a second layer disposed on thefirst layer, the second layer comprising aluminum oxide and having aplurality of unconnected pores, wherein at least a portion of theplurality of unconnected pores are at least partially filled with amesoporous silica, wherein the first layer is substantially free of themesoporous silica, and wherein at least a portion of the unconnectedpores is in fluid communication with at least one of the pores of thefirst layer.
 39. A method comprising: providing a first layer comprisinga plurality of pores; disposing a second layer on the first layer,wherein the second layer comprises a plurality of unconnected pores; andfilling at least a portion of the plurality of unconnected pores atleast partially with a filler, wherein the first layer is substantiallyfree of the filler.
 40. The method of claim 39, wherein disposing asecond layer comprises depositing a conducting layer on the first layer;and anodizing the conducting layer to convert the conducting layer intoa porous layer.
 41. A method comprising: providing a first layer havinga plurality of pores; disposing a conducting layer comprising an outersurface and an inner surface on the first layer; anodizing theconducting layer from the outer surface to convert the conducting layerinto a porous layer such that a barrier layer is present at theinterface between the first layer and the second layer; filling at leasta portion of the plurality of unconnected pores at least partially witha filler, wherein the first layer is substantially free of the filler;and removing the barrier layer to obtain a membrane structure.